Using cramer's Rule to solve for x2, i'm confused on this example!

I scanned the page out of the book, alittle bit is kinda burly but i'm confused on how they got A_2(B), where we are thinking of the system in matrix form AX = B. One verfies that detA = -127 and det[A_2(B)] = 313, so
x2 = det[A_2(B)]/det(A) = 313/-127 = -313/127;